Page 237 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

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Part III

Material Behavior

Chapter 5 and Chapter 6 give an overview of the micromechanisms of fracture in various material
systems. This subject is of obvious importance to materials scientists, because an understanding
of microstructural events that lead to fracture is essential to the development of materials with
optimum toughness. Those who approach fracture from a solid mechanics viewpoint, however,
often sidestep microstructural issues and consider only continuum models.
In certain cases, classical fracture mechanics provides some justification for disregarding
microscopic failure mechanisms. Just as it is not necessary to understand dislocation theory to
apply tensile data to design, it may not be necessary to consider the microscopic details of fracture
when applying fracture mechanics on a global scale. When a single parameter (i.e., K, J, or crack-
tip-opening displacement (CTOD)) uniquely characterizes crack-tip conditions, a critical value of
this parameter is a material constant that is transferable from a test specimen to a structure made
from the same material (see Section 2.9 and Section 3.5). A laboratory specimen and a flawed
structure experience identical crack-tip conditions at failure when the single parameter assumption
is valid, and it is not necessary to delve into the details of microscopic failure to characterize global
fracture.
The situation becomes considerably more complicated when the single-parameter assumption
ceases to be valid. A fracture toughness test on a small-scale laboratory specimen is no longer a
reliable indicator of how a large structure will behave. The fracture toughness of the structure and
test specimen are likely to be different, and the two configurations may even fail by different
mechanisms. A number of researchers are currently attempting to develop alternatives to single
parameter fracture mechanics (see Section 3.6). Such approaches cannot succeed with continuum
theory alone, but must also consider microscopic fracture mechanisms. Thus, the next two chapters
should be of equal value to materials scientists and solid mechanicians.